A switched-mode power supply forming a DC/DC converter conventionally comprises a high-frequency transformer making it possible to produce galvanic insulation and apply a transformation ratio to the voltage and the current. The transformer comprises windings forming at least one primary coil and at least one secondary coil, and a magnetic circuit allowing them to be coupled.
In addition to safety and reliability, the designers of a switched-mode power supply seek to optimize, on the one hand, the conversion efficiency, and, on the other hand, the power density.
These two objectives are however linked by the fact that, while a high efficiency makes it possible to reduce the energy consumption for a given output power, it above all makes it possible to reduce the losses generated and therefore the volume and the weight of the cooling means necessary to limiting overheating. Obtaining a high efficiency is therefore an important asset in optimizing the power density of the switched-mode power supply.
The main variable influencing the compactness of the switched-mode power supply is its switching frequency. The use of a high switching frequency tends to reduce the energy stored in the reactive elements, inductors and capacitors, and thereby reduce the volume thereof.
However, a high switching frequency has a negative impact on efficiency because of the switching losses in the switching components (transistors, diodes), which are proportional to that frequency, and on the level of losses in general. The level of losses is in fact increased because of the increase, at high frequency, in the relative significance of stray capacitances, and the increase in the electrical resistance of the conductors. The increase in the electrical resistance of a conductor passed through by a high-frequency current is due to the “skin” and “proximity” effects.
The skin effect results from the fact that a conductor, passed through by an alternating current, generates a magnetic field which, by feedback, tends to create a current neutralizing the initial current, with the consequence of “driving” the alternating current into the periphery of the conductor. Thus, the higher the switching frequency, the lower the current density at the centre of the conductor, which increases the equivalent electrical resistance. This effect exists also between two adjacent conductors, when the magnetic field created by the current in one conductor affects the distribution of the current in the second: this is then called proximity effect.
The impact of the skin and proximity effects in the transformer of a switched-mode power supply is highly detrimental.
When wanting to increase the power and therefore the current supplied by a switched-mode power supply without penalizing the efficiency thereof, the section of the conductors should be increased to reduce their electrical resistance. However, at high frequency, the current does not penetrate into the core of the conductors because of the skin and proximity effects, and the increase in the section of the conductors is ineffective.
It is therefore understood that it is complicated to increase the power supplied by a switched-mode power supply while conserving the efficiency and the compactness thereof.
To mitigate the skin and proximity effects, the state of the art is to try to compensate the ampere-turns generated between different conductors forming adjacent coils of the transformer by means of shrewd geometrical arrangements.
Among the known techniques, Litz wires can be cited, which are manufactured by weaving strands that are electrically insulated from one another, whose diameter is less than the value of the skin thickness at the switching frequency. Since each unitary strand is alternately on the inside then on the outside of the wire, the magnetic field that it creates is compensated with that of the other strands.
It is also possible to interleave the different coils of a transformer to progressively neutralize the magnetic field created by each coil.
These solutions, despite an inevitable surfeit for the first (the effective section of a Litz wire is that much less than that of a “solid” conductor of equivalent outer diameter as the unitary strands are fine) and of stray capacitance for the second, prove relatively effective for attenuating the skin and proximity effects in the windings of the transformer. However, these benefits disappear outside the windings, when connections have to be created. The losses due to the high frequencies in the connections between the windings and the components linked to the transformer are then significant, and the overall efficiency of the transformer and of the switched-mode power supply is thereby significantly reduced. This phenomenon also partly explains the differences in efficiency observed between theoretical calculations and experimental measurements, the experimental locating of these losses being very difficult to perform reliably and accurately.